This invention relates generally to initial acquisition techniques for a time division multiple access (TDMA) communication system and more particularly to those adapted for a TDMA Satellite Communication System employing a stationary type communication satellite.
Multiple access communication techniques enable a plurality of stations to communicate with each other through the intermediary of a relay station common thereto, as widely practiced in the field of satellite communication. Multiple access communications systems have previously been operated by preliminarily allocating different frequencies to the respective stations. This is called a frequency division multiple access (FDMA) communication system.
Recent development of digital techniques has given rise to vigorous attempts to employ TDMA communication techniques instead of FDMA and many experiments in this area are being performed, some of them resulting in practical applications.
In the TDMA communications, signals transmitted from respective stations take the form of an intermittent signal called "burst", which is repeated in regular periods corresponding to those of the TDMA time frame and it is required that such signals be properly synchronized so as not to overlap each other at the relay station to cause interference therebetween. To meet this requirement, a reference burst which includes a synchronizing signal as a reference for the TDMA time frame is transmitted from a reference or master station as selected from the plurality of transmitting-receiving stations participating in the communication system. The burst signals, being transmitted from the respective participant stations, are at all times controlled at the transmitting end so as to be held in a predetermined time-positional relation to the synchronizing signal included in the reference burst. It is to be noted at this point that the so-called initial acquisition technique, with which the present invention is concerned, is a technique for enabling a station, wanting to enter into communication, to insert the station's signal burst in the time slot assigned to the station. The initial acquisition technique is one of the basically important techniques required for TDMA communication systems. Also, it is required that the initial acquisition be obtained with a minimum interference with communications already in progress between other stations.
In this connection, if a station in an initial acquisition attempt can transmit such an access signal as can be received at a substantially high signal-to-noise (S/N) ratio and insert it in the station's assigned time slot, the station's burst transmission can be initiated at any time after any possible error of the access signal in position relative to the synchronizing signal included in a reference burst has been determined and duly corrected.
Accordingly, the present invention particularly relates to techniques for obtaining "rough" transmission timing information in an initial acquisition attempt at its earliest stage in order to enable a transmitting-receiving station to transmit a signal such as to be received at a substantially high S/N ratio and insert such signal in the time slot assigned to the station without allowing it to overlap any of the signal bursts being transmitted by other stations.
Such rough transmission timing information can be easily obtained in cases where the exact distance between the relay and transmitting stations is known but, in cases where the distance varies moment by moment as in the case of satellite communication, complicated means are required just to determine the station distance and techniques effective to overcome this situation are keenly demanded.
In one of the known initial acquisition techniques, a special form of access signal, which is phase-shift-keying (PSK) modulated by a pseudorandom noise (PN) or other special digital signal train, is continuously transmitted at a power level about 20 dB below the normal signal level, and the positional relationship between the signal train as received and demodulated and the synchronizing signal included in the reference burst, is determined in order to obtain the transmission timing information required.
In another known initial acquisition method, undulated carrier wave pulses of low power level are transmitted and such pulses are manually or automatically swept within the TDMA time frame in order to detect when they are positioned in the assigned time slot.
These well-known techniques are highly efficient since the communication for initial acquisition can be executed in the same frequency band as used in normal communication. However, the S/N ratio of the received access signal is extremely low because of the limited power transmission of the signal. To overcome this situation, the bit width of the PN train or the pulse width of the unmodulated carrier wave has previously been made much larger than the bit width of normal communication signals. For reception of such signal, a narrow-band band-pass filter (BPF) has been employed so that the signal may be demodulated at an improved S/N ratio.
On the other hand, it is supposed that the received signal frequency fluctuates due to a number of different factors. Namely, more or less frequency deviation may result from the arrangement of components such as a carrier wave oscillator and a local oscillator for frequency conversion from IF to RF in the transmitter, a local oscillator in the relay station, and a local oscillator used in the receiver for frequency conversion from RF to IF, and also under the Doppler effect upon signal transmission, which arises with variation in distance between the operating and relay stations.
In a typical example of satellite communication systems employing a 6 GHz/4 GHz band, the frequency deviation has been of the order of several tens of KHz, but it is expected to increase in the future as the frequency band employed is expanded to 10 GHz or over.
Under these circumstances, in order that the previous initial acquisition techniques employing a narrow-band BPF may be utilized satisfactorily, it has been necessary to additionally employ an AFC (automatic frequency control) means to enable the access signal received to pass through the narrow-band BPF and this would not only make the installation complicated as a whole, but also increase the length of time required for each initial acquisition.